BEST MEASUREMENT OF THE GRAVITATIONAL
CONSTANT.
At this week's American Physical Society Meeting in Long
Beach, Jens H. Gundlach of the University of Washington
(paper P11.3) reported a long-awaited higher precision
measurement of the gravitational constant, usually
denoted by the letter G. Although G has been of
fundamental importance to physics and astronomy ever
since it was introduced by Isaac Newton in the
seventeenth century (the gravitational force between two
objects equals G times the masses of the two objects and
divided by their distance apart squared), it has been
relatively hard to measure, owing to the weakness of
gravity. Now a group at the University of Washington has
reduced the uncertainty in the value of G by almost a
factor of ten. Their preliminary value is G=6.67390 x
10^-11 m^3/kg/s^2 with an uncertainty of 0.0014%.
Combining this new value of G with measurements made with
the Lageos satellite (which uses laser ranging to keep
track of its orbital position to within a millimeter)
permits the calculation of a brand new, highest precision
mass for the earth: 5.97223 (+/- .00008) x 10^24 kg.
Similarly the new mass of the sun becomes 1.98843 (+/-
.00003) x 10^30 kg. Gundlach's (206-543-4080,
jens@phys.washington.edu) setup is not unlike Cavendish's
venerable torsion balance of two hundred years ago: a
hanging pendulum is obliged to twist under the influence
of some nearby test weights. But in the Washington
experiment measurement uncertainties are greatly reduced
by using a feedback mechanism to move the test weights,
keeping pendulum twisting to a minimum.
See Gundlach's written summary at http://www.aps.org/meet/APR00/baps/vpr/layp11-03.html
figures at www.aip.org/physnews/graphics

GRAVITY HAS BEEN MEASURED AT THE SUB- MILLIMETER
SCALE for the first time. Gravity has of course
long been studied over planetary distances but is more
difficult to gauge at the terrestrial scale, where
intrusive electric and magnetic fields, many orders of
magnitude stronger than gravity fields, can be
overwhelming. Nevertheless, Eric Adelberger and his
colleagues at the University of Washington have managed
to measure the force of gravity over distances as small
as 150 microns using a disk-shaped pendulum carefully
suspended above another disk, with a copper membrane
stretched between them to help isolate electrical forces.
(This experiment should not be confused with another
University of Washington effort in which the
gravitational constant is measured with higher precision.
Adelberger (206-543- 4294, eric@gluon.npl.washington.edu)
presented one of several talks at this week's APS meeting
in Long Beach, California devoted to short-range gravity,
a subject which has suddenly attracted much theoretical
and experimental interest owing to a relatively new model
which supposes the existence of extra spatial dimensions
in which gravity, but not other forces, might be
operating. According to Nima Arkani-Hamed of LBL
(arkani@thsrv.lbl.gov, 510-486- 4665) this is why gravity
is so weak: it dilutes itself in the extra dimensions. In
other words, ordinary particles are tethered to our
conventional spacetime, or "brane," while
gravitons are free to roam into otherwise unseeable
dimensions. One implication of the model, testable with
tabletop experiments such as Adelberger's, is that the
gravitational force might depart from Newton's inverse
square law (gravity inversely proportional to the square
of the distance between two objects) at close range.
Adelberger did not observe such a departure at distances
down to tenths of a millimeter and will continue to
explore even shorter distances. Another interesting
implication of the model introduced by Arkani-Hamed (and
others; see preprint hep-th 9803315) two years ago is
that the unification of the four known forces would not
necessarily occur at energies as high as 10^19 GeV but
possibly at energies as low as 10^4 GeV, an energy scale
within reach of the Large Hadron Collider under
construction at CERN. Extra dimensions could, for
example, manifest themselves in proton- proton smashups
as an apparent disappearance of energy, implying that
some of the collision energy had been converted into
gravitons (the particle form of gravity) which then
disappear into the extra dimensions. The gravitons
produced in this way might come back into our
conventional world of 3 spatial dimensions and decay into
two photons. Physicists have already looked for this kind
of event. Gregory Landsberg of Brown University
(401-863-1464; landsberg@hep.brown.edu) reported that at
the D0 experiment at Fermilab some energetic two-photon
events have been observed (including one in which the
energy of the photons added up to 574 GeV, representing
the highest composite mass ever seen in the D0
experiment) but not often enough to constitute evidence
for extra dimensions. In fact this shortage of events has
been translated into a lower limit of 1300 GeV for the
energy at which a prospective unification of the forces
could take place.